The light emitting diode (LED) is based on a forward biased p-n junction. LEDs have recently reached high brightness levels that have allowed them to enter into new solid-state lighting applications as well as replacements for high brightness light sources such as light engines for projectors and automotive car headlights. These markets have also been enabled by the economical gains achieved through the high efficiencies of LEDs, as well as reliability, long lifetime and environmental benefits. These gains have been partly achieved by use of LEDs that are capable of being driven at high currents and hence produce high luminous outputs while still maintaining high wall plug efficiencies. Solid state lighting applications require that LEDs exceed efficiencies currently achievable by alternative fluorescent lighting technologies.
The total efficiency of LEDs can be quantified by three main factors, namely internal quantum efficiency, injection efficiency, and extraction efficiency, the latter of these being a particular practical problem. However, injection efficiency and extraction efficiency are not entirely unrelated. For example, if an improvement in optical management can be achieved it may then be possible to accommodate larger ohmic contacts, thereby leading to improved injection efficiency.
One of the main limiting factors reducing the extraction efficiency in LEDs is the emitted photons being totally internally reflected and trapped in the high refractive index of the epi-material. Vertical light emitting diodes typically employ metal substrates and top contact regions. These characteristically introduce optical loss to trapped waveguide modes residing in the light emitting region. Current top surface roughening techniques are employed to extract the maximum amount of light on the first pass of light incident on the top surface. However, if light is multiply scattered by the bottom metal reflector then loss is introduced to the trapped waveguide mode. These trapped waveguide modes propagate in the LED structure until they are scattered, escape or reabsorbed. The thickness of the LED structure determines the number of modes that can be set up.
Many methods have been successfully employed to improve light extraction in LED heterostructures. These include shaping LED die, as described in U.S. Pat. No. 6,015,719 and U.S. Pat. No. 6,323,063, flip-chip mounting of LEDs as described by Wierer et al. in Appl. Phys. Lett., 78, Pg. 3379, 2001 as well as roughening of the top surface as taught by Schnitzer et al in Applied Physics Letters 63, 2174, 1993, and omnidirectional reflectors as suggested by Fink et al. in Science vol. 282, Pg. 1679, 1998. Other suggested methods include the use of periodic texturing on at least one interface of the structure to improve light extraction out of the light emitting region as suggested in U.S. Pat. No. 5,779,924.
To provide light emitting devices with high current and thermal driving capabilities the vertical type n-p contact configuration in GaN material systems has been recently adopted. Such examples have been disclosed in U.S. Pat. No. 6,884,646 and U.S. Patent 20060154389A1. However, one major drawback with such vertical type light emitting structures is the existence of optically lossy metal contacts in the close vicinity of the light emitting heterostructure. Trapped modes in the high index light emitting device typically undergo multiple internal reflections. The photons reflected at the interface between the metallic contact surface and the heterostructure material experiences large losses and hence reduces the total light output of the light emitting diode.
One of the fundamental ways to reduce optical losses in light-emitting devices is the employment of optical reflectors. Because multiple reflections occur in LED structures, mirror losses should be kept at a minimum by using reflectors with near-perfect reflection characteristics. Such near-perfect characteristics of a reflector include (i) high reflectivity, (ii) omni-directionality, (iii) broad spectral range of the high-reflectivity band and (iv) electrical conductivity for current-injected structures. There are several reflectors used in current LED structures including the metal reflector, the Distributed Bragg Reflector (DBR) and the Omni Directional Reflector (ODR).
Metal reflectors are electrically-conducting reflectors capable of reflecting visible light over a wide range of wavelengths and incident angles, i.e. showing omni-directionality and broad spectral width. However, total integrated reflectivity across all incidence angles of a metal reflector on a semiconductor is typically at most 96%. For example, Ag, which shows highest reflectivity among metals at visible wavelengths, exhibits normal-incidence mean reflectivity on GaN of 95.7% at 455 nm.
A DBR is a periodic structure with a unit cell of two dielectric layers having different refractive indices ni and quarter-wavelength thicknesses. A DBR can be designed to have a reflectivity as high as 99% at a certain centre wavelength for normal incidence. However, the DBR reflectivity depends on the incidence angle θ so that the stop band shifts towards shorter wavelengths for increasing θ without changing its spectral width. As a result, DBRs become transparent for oblique angles of incidence. In addition, the reflectivity of DBRs also depends on the polarization of the incident light. Therefore, the overall reflectivity, (TM mode+TE mode)/2, significantly decreases at oblique angles. Typically the number of layers needed is in the 10-50 range and therefore the applicability of DBRs in LEDs is limited due to the electrically insulating and thermally-resistant nature of dielectric layers.
In U.S. Pat. No. 6,784,462 the use of an omni-directional reflector (ODR) is proposed. This single dielectric electrically insulating layer is disposed between the light emitting region and the lower conductive region and having a plurality of electrical conductive vias contacting the lower light emitting region and an electrical contact. It is typically an object of vertical light emitting devices to provide good electrical and thermal conduction, a single dielectric layer will not provide true omni-directional reflectivity and angles residing within the escape cone formed between the light emitting medium and the dielectric layer will experience a reflection at the metal contact boundary which will introduce optical loss.
In Schubert et al APPLIED PHYSICS LETTERS 90, 141115, Apr. 6, 2007 a Conductive DBR structure is described using a single conductive material Induim Tin Oxide (ITO) deposited at an angle to provide both low and high refractive index layers (refractive index contrast 0.4), for the DBR and this achieved a theoretical reflectivity of 74%. This technique has the advantage of conductivity but at the expense of index contrast.
There are many applications for solid state lighting, and one of the largest emerging applications for this type of light emitting module is in the general lighting market. Solid state lighting offers many benefits over conventional lighting techniques such as incandescent, halogen and compact fluorescent lighting, and these benefits include small form factor, environmental, high efficiency, linearly dimmable, instant on, very long lifetime, tunable colour temperature, simple interface and control as well as capability of high switch speeds. Other markets that that already benefit from the advantages that solid state lighting offers include architectural, medical and signage applications.
Back Light Units (BLU) for LCD panels are key elements to the performance of an LCD panel. Currently most LCD panels employ compact cathode fluorescent light (ccfl) sources, however, these suffer from several problems such as poor colour gamut, environmental recycling and manufacture issues, thickness and profile, high voltage requirements, poor thermal management, weight and high power consumption. In order to alleviate these problems LCD manufacturers are implementing LED BLU units. These offer benefits in improved light coupling, colour gamut, lower power consumption, thin profiles, low voltage requirements, good thermal management and low weight.
Another application for LED modules is in light engines for front and rear projectors. Conventional High Intensity Discharge (HID) type projector light engines have always been hindered by large size, low efficiency and short lifetime resulting in slow adoption into consumer markets.
Thus, there are a wide range of applications for LED modules, if the problems limiting the efficiency can be alleviated. There is therefore a need for a more efficient design of LED, which can achieve the performance levels required for this type of solid state lighting device to replace more conventional sources.